This invention relates to a process for manufacturing a carbonaceous substance and, more particularly, for manufacturing an isotropic carbonaceous substance having a high density.
It is an obvious fact that the fundamental properties and the structure of a carbonaceous substance are for the most part determined at an early stage of carbonization which is performed at a temperature between 300.degree. C. and 500.degree. C. That is to say, the fundamental properties of the carbonaceous substance are determined beginning with the stage of an intermediate product, the so called mesophase, which can be observed with polarized light by means of a polarizing microscope at an early stage of carbonization.
The properties of the carbonaceous product apparently depend on the condition of thermal treatment under which the intermediate product is produced. For example, in order to obtain a carbonaceous substance having good graphitization, i.e., so called soft carbon, it is necessary to select a raw material from which a spherical liquid crystalline intermediate product (mesophase) [A] is made at an early state of carbonization after which a substance having an ordered planar structure of condensed aromatic rings [B] is formed by the coalescence of [A] and finally such that a substance having a large fibrous texture [C] is made by the growth of [B]. In addition, proper selection is necessary of the conditions of the thermal treatment. For the aforesaid reason, in order to obtain a carbonaceous substance with the desirable structure and properties for graphitization, it is necessary to select the raw material and the conditions of thermal treatment thereof satisfactory to make the suitable intermediate product (mesophase).
The following methods have generally been proposed as processes for manufacturing an isotropic carbonaceous substance of high density. In accordance with one of these methods, petroleum coke is crushed into particles of 10 .mu. diameter or less, the crushed coke is molded with a binder, the molded body is treated thermally followed by impregnation with a pitch oil, and the resulting impregnated article is then treated thermally in repeated fashion. Thus, this method requires a very complex process as described above, and therefore, the manufacturing cost is very high. Recently, another process of manufacturing an isotropic carbonaceous substance with high density has been developed, which comprises treating coal tar pitch or residues from vacuum distillation of petroleum oil at a temperature between 300.degree. C. and 500.degree. C. for a few hours to produce an intermediate product (mesophase), extracting the said mesophase at an early stage of carbonization with a solvent having a high boiling point and then carbonizing or graphitizing the said mesophase after molding without a binder. But, in this method, fine particles of the mesophase obtained by the extraction are bonded to each other by the fusion of the fine particles during the carbonization step. Thus, a countermeasure to protect against this bonding is necessary. For example, a method is known wherein carbon black having finer particles than those of the intermediate product (mesophase) is coated on the surface of the intermediate product (mesophase), and another method is known wherein a matrix portion attached to the intermediate product is washed several times in order to remove the matrix portion, which causes the particles to separate. But, according to the above methods, a complex process is necessary and has the disadvantage that the yield of product based on the raw material is low.
Also, mosaic crystal having a small structural unit is observed with polarized light under a polarizing microscope at an early stage of carbonization of a coal such as one having low rank of coalification, for example, Akahira coal and Taiheiyo coal in Japan. The presence of ashes in the coal is considered as one of the causes for the formation of mosaic crystal. Moreover, it is considered as another important cause of forming mosaic crystal that the bridge formation by hetero atoms (O, N and S) in coal obstructs the planer orientation of the aromatic rings. It is also true that the lower rank of coalification, the smaller is the unit of mosaic crystal at the early stage of carbonization. This is considered to be attributable to the fact that the lower the rank of coalification, the richer is the content of hetero atoms (O, N and S) in the coal, and the richer is the aliphatic substance and, therefore, bridge formation is greater.